This invention pertains to the purification of vegetable oil, and particularly to the degumming of such oil.
Oil extracted from vegetable matter such as soy beans, cottonseeds, peanuts and the like has many uses, particularly as food for animals and humans. While such oil may be extracted mechanically by pressing the seeds or beans, it is common to extract the oil with a solvent such as hexane. Frequently, oil is extracted from the bean residue after pressing. It is this solvent extraction method to which the present invention applies.
While most of the solvent can be removed from the oil in an evaporator by heating it to about 200.degree.-250.degree. F., it is necessary to remove the last traces of solvent by steam stripping. This is conventionally done by injecting steam into the bottom of a stripping column while maintaining a vacuum of from 24 to 29 inches of Hg at the top. The vacuum is expressed as inches of mercury less than atmsopheric pressure, i.e., a negative gauge pressure.
The crude oil thus extracted still contains impurities which must be removed for must uses. Among these impurities are the so-called gums or phosphatides often referred to, particularly in the case of soy bean oil, as lecithin. The amount of these impurities differs in different oils, ranging from 0.1% or less up to 3% or more in oil such as soy bean oil. Lecithin removed from vegetable oil is a useful product. It may be used as such in animal feed and when dried and purified it may be used in human foods such as margarin, candy, artificial creamers, and many others.
There are a number of methods for removing the phosphatides or "gums" from crude vegetable oil. One of the methods is to add water to hydrate the phosphatides to precipitate them as a sludge. The hydration is conventionally done by mixing water with oil in a mixer where intimate contact is maintained until gum precipitates. The hydrated gums are heavier than the oil and can be separated from the oil on the basis of density of settling or in a device known as a degumming centrifuge. A certain amount of water is taken up by the oil in the steam stripping operation, usually about 0.1% to 0.5% based on the weight of oil. All percentages stated in this specification and the appended claims are on a weight basis. Because of the high vacuum and high temperature used in this step, insufficient water is taken up to fully hydrate the phosphatides. Generally, an amount of water about equal in weight to the content of the phosphatides or gums in the oil is needed. For example, about 1.5%-3% water would be needed to precipitate gums from soy bean oil. Accordingly, it is necessary to add water to the stripped oil and to hold the mixture in a mixer and maintain good contact between the oil and water phases for a sufficient time for hydration of the gums to take place. In addition, it is necessary to add water in excess of that needed to precipitate hydrated gum or sludge so that the "gum" or lecithin or sludge removed in the degumming-type centrifuge will have sufficiently low viscosity to discharge from that apparatus as a separate phase. As will be discussed below, the lecithin or gum removed from this type centrifuge typically contains about 30% to 45% water.
By the term "degumming-type centrifuge" is meant a centrifuge of the type commonly used in this industry: for example, a Model OSM 8004 centrifuge manufactured by Westphalia wherein two material of different specific gravities are separated and the apparatus is designed to discharge the higher density material at a relatively low viscosity, i.e., a viscosity less than 5000 poises.
The products of this centrifuging step are degummed oil and wet gum. The latter contains roughly equal parts of oil, lecithin, and water. This material must be immediately dried, used, or processed, or it will ferment within a day or so. Fermented or spoiled gum is a viscous, useless material which is expensive to dispose of.
In an alternative type of processing, the crude oil can be taken without prior degumming directly to a caustic refining step where caustic (e.g., sodium hydroxide or sodium carbonate) is added to remove the free fatty acids from the oil as soaps. The caustic phase, which is known as soapstock and includes the hydrated phosphatides or gum, is separated from the oil in a refining centrifuge as the heavy phase. The soapstock is neutralized with acid (e.g., sulfuric) and washed with water to produce waste water, which must be treated before it can be disposed of, and a dark, high free fatty acid product known as "acidulated oil." This latter product finds use as a cattle feed, as do the extracted seed flakes produced in the original extraction of oil with solvent. When vegetable oils are processed in this manner, lecithin cannot be recovered because it is destroyed by the caustic and acid treatment.
In still another alternative processing method, the hydrated oil can be centrifuged to remove the gums, and the degummed oil is then sent to a caustic refining step. This method has the advantage that less caustic and acid are used and there is greater recovery of usable products.
The present invention is an improvement over the conventional methods of degumming and refining vegetable oil by hydration and centrifuging which simplifies the process by eliminating certain steps and which additionally produces an improved lecithin product.
The art of purifying vegetable oils is well set forht in an article by Roy A. Carr in the Journal of American Oil Chemists Society, Volume 53, June 1976, starting at page 347. The terms "hydrated gums", "sludge", and "lecithin" are used interchangeably when used to refer to a precipitated phase or a separate phase within the oil that can be separated by gravity.